1. Field of the Invention
The present invention relates to an optical connector for connecting optical fibers on their ends which are used as a transmission medium in optical communication and to a "ferrule" which is a component part of an optical connector used to sheath the ends of optical fibers.
2. Description of the Prior Art
Optical fibers with excellent transmission characteristics such as high band and low loss have been extensively used recently as a medium in transmission of data and photo-energy. Along with such development, there is keenly felt demand for a method for connecting optical fibers of the diameter of ca. 0.1 mm which are often used for high speed data transmission with low coupling loss at low cost.
Since optical fibers are thin and fragile, connecting optical fibers requires technology more sophisticated than in electric connection. More specifically, in order to connect optical fibers at a coupling point at lower energy loss, it is necessary to align optical axes of two optical fibers at precision of several microns so as to have less gap between interfaces thereof. (This is referred to as alignment of optical axes.)
Conventional methods for connecting optical fibers are outlined, for example, in Technical Stuff of CSELT: "OPTICAL FIBER COMMUNICATION," pp. 541-643, especially pages 571-602 1980, McGraw-Hill New York. They are roughly classified into two: the one which permanently connects optical fibers and the other which employs a connector to allow connection and disconnection of optical fibers. As the present invention is intended for connection between equipments, it relates to the latter category method which connects optical fibers by means of a connector in a manner to allow detachment.
Structure of such optical connectors in the prior art generally comprises ferrules and a sleeve such as shown in Japanese Patent Publication No. Sho 55-22707. The ferrule is tubular and has a circular section which is axially bored a throughhole having a slightly larger diameter than the outer diameter of an optical fiber for receiving the same. A sleeve is a cylindrical member for accurately abutting two ferrules. Optical fibers are connected by inserting terminals of two optical fibers into throughholes on the ends of ferrules respectively, aligning and fixing end faces thereof and the ferrules on the same plane, and inserting the ferrules into a sleeve to abut end faces of the ferrules on each other.
In such structure of optical fiber connection, if there is angular or axial deviation between the optical fibers, coupling loss is increased. In order to achieve excellent connection at low coupling loss, axial or angular deviation should be kept to minimum. Conventional ferrules are designed as rigid bodies to avoid distortion which may lead to axial and regular deviation of the optical fiber axis to thereby prevent any increase in coupling loss. Favorable characteristics of optical fibers such as smaller diameter and flexibility are therefore not fully utilized.
An example of conventional ferrules is shown in FIG. 1. A ferrule 1 is made of stainless steel and comprises a cylindrical part 2 and a flange 3. The diameter D of the cylindrical part 2 (hereinafter referred to as "outer diameter of a ferrule") is 2.5 mm and the length L of the cylindrical part from the flange 3 to an end 4 of the ferrule (hereinafter referred to as "length of a ferrule cylinder) is 8.0 mm. A capillary 7 made of ceramics with a precision bore hole 6 is fitted into the tip end of the cylindrical part 2. The spring constant when fixed as shown in FIG. 2 and deformed by the stress applied on the tip end 4 becomes approximately 20 kg/mm, suggesting the high rigidity of the ferrule. The weight applied at breaking is ca. 10 kg. Displacement at the tip end even at this time is less than 0.1 mm. FIG. 3 shows another type of ferrule of a similar shape to the one shown in FIG. 2. However, this ferrule is not provided with a capillary and made of plastics (PPS resin containing a reinforcing agent). With this ferrule, the spring constant is several tens of kilograms/mm and the weight at rupture is several kgs. The displacement at the tip end is less than 0.5 mm at the time of rupture.
As mentioned above, a conventional ferrule is characterized in that it is rigid and resistive against deformation. It is therefore necessary to position two ferrules precisely not only at a coupling point but also along the whole structure thereof when optical axis are aligned. When the two ferrules each inserted with an optical fiber are abutted as shown in FIG. 4 at a coupling point, a plug 10 cannot be inserted into an adaptor 11 if a ferrule 1' is fixed into the plug 10 at a location deviated from the central axis of a sleeve 14 by a distance H at which the central axes of the end faces 9 and 9' of the ferrules 1 and 1' should be aligned. A forced insertion may result in damage of the ferrule 1' or of the plug or the adaptor as the ferrule 1' is not easily bendable and the bending stress on the ferrule 1' exceeds the yield point. In either case, alignment of optical axes cannot be achieved effectively.
Conventional plugs and adaptors therefore needed high precision in manufacturing. Alternatively ferrules are raised from the plug by a spring while securely held by a sleeve with enough force in order to avoid any axial and angular deviation between the ferrules. However, if the retentive force is increased, friction between the sleeve and the ferrules increased, and the force needed for pulling out the plug unavoidably increases. At the same time, friction between the sleeve and the ferrules might cause damage. If an increased force should be applied for pulling out the plug, that would present a problem in the development of multicore optical connectors. In addition, dust generated from the friction contributes to increasing of the loss.
Optical fiber connectors are expected to be packaged at higher density in near future and for that purpose, ferrules which are the basic component of a connector should be reduced in size. The prior art ferrules needed high strength to resist against outer forces rigidly as they are not easily bendable. Due to this requirement in strength, conventional ferrules cannot be reduced in outer diameter beyond a certain limit. The length of a sleeve cannot be shortened very much as coupling of ferrules at low loss requires the length to be several times of the outer diameter of the ferrules. The conventional rigid ferrules therefore cannot be reduced in size beyond a certain value.
Further, as the prior art ferrules are rigid, the structure of optical axis alignment using the flexibility of optical fibers which is an advantage of the slice method is not employed. Such method is proposed, for instance, in Japanese Utility Model Application laid-open No. Sho 53-93241. The method is for connecting optical fibers permanently by pressing an optical fiber against a corner with a bending stress generated from bending of the fiber and accurately aligning the optical axis of optical fibers with relative positional relation between the corner and the optical fiber. Japanese Patent Application laid-open No. Sho 57-139716 discloses a connection method which employs the flexibility of optical fibers. That is the method of pressing each optical fiber against a v-shaped grooved element with a retention spring for alignment of optic axes. The method allows to absorb mismatching of optical fibers in the axial direction with the flexibility thereof and as v-shaped groove elements are of low cost and the dimensional precision used in the method is not so high, greater economic advantage is expected.
The methods using the flexibility of optical fibers have been known in the prior art, but as fragile optical fibers are used naked in all of these methods, the methods cannot be employed for optical fiber connectors which connect optical fibers in a detachable manner. Moreover, as the prior art ferrules are not easily bendable, the methods are hardly applied.
An object of this invention is to provide a ferrule which can be precisely positioned and aligned on an end face thereof with another ferrule at a coupling point by absorbing mismatching between the two ferrules with suitable deformation thereon even if they are fixed at axial positions which are missligned from each other.
Another object of this invention is to provide a ferrule which is not broken or damaged even if it is subjected to deformation lby distributing outer force evenly with elastic deformation.
Still another object of this invention is to provide a ferrule which needs less force for aligning optical axes by making the ferrule per se flexible.
Still another object of this invention is to provide a ferrule which is advantageous economically.
Still another object of this invention is to provide a ferrule of a smaller size which can be applied in a high-density optical connectors.
Still another object of this invention is to provide a connector which is applied with an optical axis alignment mechanism utilizing aforementioned flexibility.
Still another object of this invention is to provide an optical connector which needs smaller force for insertion as well as pull-out.